Hydrophobic and Elastic Forces Experienced by a Series of Pyrene End-Labeled Poly(ethylene oxide)s Interacting with Sodium Dodecyl Sulfate Micelles

Abstract

A series of pyrene end-labeled poly(ethylene oxide)s (Py2-PEO(x) with x equal to 2, 3.4, 5, 6, 7.8, and 10 kg mol–1) were synthesized, and their interactions with SDS micelles were characterized by steady-state and time-resolved fluorescence. Pyrene excimer formation (PEF) indicated that both pyrenyl ends were located in the same SDS micelle whereas the absence of PEF reflected bridging of two SDS micelles by a Py2-PEO molecule having its pyrenyl end groups in separate micelles. PEF efficiency was characterized from the excimer-to-monomer fluorescence intensity ratio (IE/IM) which was determined as a function of PEO chain length and salt concentration. More detailed information about the PEF process was obtained by fitting the monomer and excimer fluorescence decays globally according to the model free analysis (MFA). MFA yielded the average rate constant ⟨k⟩ of PEF and the molar fraction (ffree) of pyrenyl labels that did not form excimer, thus reflecting the extent of bridging undergone by the Py2-PEO constructs. Bridging was inferred for the two longest Py2-PEO constructs but not for the shorter Py2-PEO(2K) sample, since ffree was large and small for the former and latter constructs, respectively. The most interesting behavior was observed for Py2-PEO(6K) where a substantial increase in ffree was detected upon addition of a minute amount of NaCl. The Py2-PEO(6K) construct marked the boundary between the regimes where bridging would or would not occur with or without salt, respectively. This observation led to a quantitative analysis of the energies involved in restraining the two end groups of a Py2-PEO construct in the same SDS micelle and in extending a PEO chain to bridge two SDS micelles occupied by one pyrenyl end of a Py2-PEO construct. The theoretical energy calculations were found to predict fairly accurately the ffree trends obtained experimentally by the MFA of the Py2-PEO fluorescence decays.